[REQ][typescript-angular] new config option to generate all model properties as readonly

The current typescript-angular generator somehow misinterprets the meaning of the OAS readOnly field of a property definition in a schema object. Please, see the actual definition of this attribute from the OAS 3.0.3 spec.

Instead, the generator treats schema object properties marked with readOnly: true field as readonly marked typescript properties, which makes those properties immutable (please, see Typescript offical doc).

The following code is an excerpt from the current generator template, which implements the described behavior:

https://github.com/OpenAPITools/openapi-generator/blob/77687a8ea59f97519731f345308d3615e2f6e500/modules/openapi-generator/src/main/resources/typescript-angular/modelGeneric.mustache#L8

The OpenAPI spec doesn't tell anything about immutability, it just speaks about the property presence in the DTO. As a matter of fact, the immutability of a property is a desired behavior in request DTOs too.

My proposal is to add a new config option for the angular-typescript generator which instructs the generation process to mark all the properties of the DTOs as readonly, whether they are "read" or "write" DTOs in the OpenAPI spec, and ignore the readOnly OAS field altogether.

Start with modules/openapi-generator/src/main/resources/typescript-angular/modelGeneric.mustache and compare its handling of the OAS readOnly field with the requested behavior. Trace how typescript-angular generator options are declared, then verify that the new option makes every generated DTO property readonly while ignoring readOnly.